Aphid Alert 2001, No. 6, July 27

Aphid captures, week ending July 27

The first capture of a winged green peach aphid occurred this week at Baker, MN. In 1999 and 2000, we started catching winged green peach aphids in mid-June. Indications are that green peach aphid numbers will remain low in 2001 throughout the northern Great Plains. Green peach aphid do not survive outdoors in this region, even as eggs. Each summer, populations are restablished by founders arriving from southern locations where overwintering occurs. This year, there have been relatively few wind events favorable for the long-distant transport of green peach aphids to the northern Great Plains (see MinDex in Aphid Alert no. 5). We have found green peach aphid on canola and other brassicas, but numbers are low compared to the past three years. In some urban locations, e.g., St. Paul and Duluth, we have found green peach aphid to be abundant in gardens suggesting introduction with transplants from glasshouses or shipped from other locations. At Rosemount, MN, a heavy infestation of green peach aphid has developed in experimental plots of canola treated with aldicarb (Temik). This outbreak suggests that this population may be resistant to aldicarb, or that the drought conditions prevailing at Rosemount have hampered uptake of the insecticide. During the past two weeks, green peach aphid nymphs have been detected on potatoes at several locations in the Red River Valley. Presumably, they are present in all potato fields, but colonies appear to be only recently established. Infestations seldom extend more than a few rows into the field. Spraying the outside rows (e.g., 20 rows or a single pass of an airplane) of seed potato fields with methamidophos (Monitor), pymetrozine (Fulfill), imidacloprid (Provado or Leverage), or thiamethoxam (Actara) should be very effective at this time.

Aphid flight activity of other aphid species increased considerably over the previous week, but continued to be less than on the same date last year. Most abundant were aphids associated with corn and small grain cereals. Most common were bird-cherry oat aphid, Rhopalosiphum padi, and corn leaf aphid, Rhopalosiphum maidis. Small grains are starting to ripen across northern Minnesota and North Dakota, so we expect flight activity of species associated with these crops to increase sharply over the next 2-3 weeks. Cereal aphids are potential vectors of PVY. Foliar insecticides are seldom effective in preventing the spread of PVY or other non-persistently transmitted potato viruses. But minerals oils can disrupt the acquisition and transmission of non-persistently transmitted viruses such as PVY and provide at least partial protection. In experiments we conducted in 2000, spread of PVY was reduced 40% by weekly application of 2% mineral oil, Aphoil (sold by Agsco). We did not observe phytotoxicity even when Aphoil was applied as a 4% suspension, but caution should be excised when using higher rates or when mixing with other pesticides.

Soybean aphid, Aphis glycines, was found this week in the Baker, MN suction trap. The nearest known field infestation of soybean aphid is at St. Paul, MN, 200 miles to the southeast. Many soybean fields in south central and southeastern Minnesota are heavily infested with soybean aphid. Often nearly every terminal is infested, some with hundreds of aphids, and most developing into winged morphs.

Notes:

Locations for which data were not available when this page was created will be updated as results are received.

One suction trap and two pan traps are operated at each location, except at Crookston, Rosemount and Thief River Falls which have more. Data are reported as total aphid captures per three traps.

In 2001, only one green peach aphid has been captured in the first 7 weeks of trapping. The usual pattern in previous years has been for aphid captures to increase steadily throughout July and peak in early to mid-August. However, in most locations we are finding very few green peach aphids colonizing alternative and weed hosts, e.g., canola and wild mustard.

Flight activity of other aphid species increased over the previous week, but numbers of potential vectors of PVY continue to be less than on the same date last year. Small grains are rapidly ripening. We anticipate the flight activity of cereal aphids will increase considerably in the next 2-3 weeks. Most canola fields are now flowering. Most canola fields do not appear to be supporting large populations of green peach aphid, but we observed one heavily infested field at Rosemount, MN. Soybean aphid, Aphis glycines, is reaching high densities on soybean in the southern half of Minnesota. Soybean aphid appears to be tolerant, if not resistant, to a wide range of insecticides. A sizable proportion of winged morphs are present on infested plants. The potential of this species as a vector of potato viruses is not known.

Means followed by same letter are not significantly different (p>0.05), Duncan's MRT.

Insecticides do not prevent spread of PVY. PVY transmission is primarily by winged aphids and occurs within the first one or two feeding probes after virus acquisition. Mineral oils have been used successful to limit spread of PVY and other plant viruses transmitted by aphids in a non-persistent manner. Aphoil (sold locally by Agsco) is a mineral oil available to potato growers. In 2000, we compared 3 rates Aphoil (1, 2, and 4%) and 2 aphidicides in a replicated field trial. Mineral oils are usually used at rates of 1-2%. Higher rates are often reported to cause phytotoxicity, but we observed no phytotoxicity using Aphoil at the 4% rate. Aphoil appears to be compatible with most fungicides and insecticides commonly used on potato.

Late Blight Alert!

Minnesota: No late blight has been reported in North Dakota or Minnesota yet. Growers can be complimented on keeping up on their spray schedules and excellent job of cull pile disposal this year. As of today, July 27th 2001, a late blight alert warning has been issued for Freeborn County and southern MN, including the towns of Waseca and Hollandale. ALL RED RIVER VALLEY SITES, as well as all central lakes region sites have also been alerted to favorable conditions for late blight. The following MN locations have also attained late blight alert status: Perham, Staples (July 20), Baudette, Crookston, Eldred, Felton, Karlstad, Stephen, Williams (July 16 & 18), Brooten, Foxhome (July 9), Humboldt (July 4), Warren (July 3), and Little Falls, Park Rapids (June 15). The weather has been favorable for late blight to spread. Please take the appropriate precautions and scout your fields frequently.

North Dakota: Cumulative severity values as of July 25 for irrigated sites ranged from 44 (Linton) to 78 (Robinson), and for nonirrigated sites from 47 (Cando, ND and Stephen, MN) to 60 (Humboldt, MN). Call NDSU Potato Late Blight Hotline at 1-888-482-7286.

Wisconsin: Cumulative severity values as of July 25 ranged from 40 at Hancock to 102 at Plover. Late blight is currently confined to an area of central WI that stretches from Coloma to Plover. The extent of the area is a concern for all growers in this area of the state. At this point in the season, no late blight has been reported from northwest, northeast, southwest, south-central or southeast WI. Growers continue to spray production fields on 5-7 day intervals. Now that weather conditions have returned to a more normal pattern with periodic rainfall and somewhat cooler temperatures, we can expect an increase in disease pressure. See Wisconsin web site for recommendations for your area.

Michigan: Late blight has been found in Michigan. Cumulative severity values are generally high and high rates and short spray intervals are recommended (see Michigan web site for recommendations for your area).